Most commercial aluminum alloys are produced by the direct chill (DC) casting process. The cast aluminum alloys (ingots) are then processed via hot and/or cold rolling into sheets or plates. Cylindrical ingots (billets) are often processed into extrusions. In the DC casting process molten aluminum flows into a short, rectangular, or circular water-cooled mold ring, which is initially closed by a plug (called a starter or bottom block) on a movable ram. The molten aluminum freezes against the bottom block and forms a solidifying shell on the mold surface. The ram is then steadily withdrawn, pulling the bottom block and the shell with it. As the shell exits the bottom of the mold, cold water is sprayed directly on it for cooling. In this manner, a cast ingot of a desired length is produced. The DC casting process is a semi-continuous process and is widely used to produce rectangular or cylindrical (billets) ingots which are subsequently rolled into sheet or plate. Billets are subsequently formed into products such as extrusions.
Unfortunately, during the DC casting process a number of defects are introduced, primarily due to the non-uniform, high rate of heat removal imparted by the direct contact of the bottom block or flowing water with partially solidified ingot. These defects increase the cost and reduce the yield of the DC casting process.
Accordingly, it is desirable to control the heat removal process and thereby to reduce the extent of surface defects. Of specific interest in this invention is the freezing of the meniscus at the top of the melt.